org-mac-mail-link.el – Create and handle links to the selected Mail.app message

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;;; Capture template for the currently selected Mail.app message
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(defun org-mac-mail-link-get-selected-message-subject
    ()
  (with-temp-buffer
    (call-process
     "osascript" nil t nil
     "-e" "tell application \"Mail\" to get subject of item 1 of (selection as list)")
    (buffer-substring-no-properties (point-min) (- (point-max) 1))))

(defun org-mac-mail-link-get-selected-message-id
    ()
  (with-temp-buffer
    (call-process
     "osascript" nil t nil
     "-e" "tell application \"Mail\" to get message id of item 1 of (selection as list)")
    ;; This additional encoding specifically of =/= is because Mail.app
    ;; claims to be unable to find a message if it's ID contains unencoded
    ;; slashes.
    (browse-url-url-encode-chars
     (buffer-substring-no-properties (point-min) (- (point-max) 1))
     "[/]")))

(defun org-mac-mail-link-get-link-string
    ()
  (let ((subject (org-mac-mail-link-get-selected-message-subject))
        (message-id (org-mac-mail-link-get-selected-message-id)))
    (org-link-make-string (format "message:%s" message-id)
                          subject)))

(defun org-mac-mail-link-get-body-quote-template-element
    ()
  (let ((body (setq body (with-temp-buffer
                           (call-process
                            "osascript" nil t nil
                            "-e" "tell application \"Mail\" to get content of item 1 of (selection as list)")
                           (buffer-substring-no-properties (point-min) (- (point-max) 1))))))
    (format "

  #+begin_quote
%s
  #+end_quote"
            (string-join
             ;; Remove duplicate empty lines
             (seq-reduce
              (lambda (acc next)
                (if (string= next (or (car (last acc)) ""))
                    acc
                  (append acc (list next))))
              ;; Indent each line by two spaces for inclusion in the quote
              (mapcar (lambda (string)
                        (let ((string (string-trim string)))
                          (if (string= "" string)
                              string
                            (format "  %s" string))))
                      (split-string body "\n"))
              '())
             "\n"))))

(require 'org-capture)

;;; You may also wish to use the Customize interface for this variable
;;; which is quite nice.
(setq org-capture-templates
      ;; These 2-item entries are only necessary if you want to nest the
      ;; capture template under a keychord.
      '(("t" "TODO")
        ("tc" "TODO Current")
        ("tcm" "TODO Current Mail" entry
         ;; If you maintain your TODO list in a single file this will
         ;; place the resulting org-capture template expansion under the
         ;; 'Inbox' heading. You may want to modify this.
         ;;
         ;; The resulting heading looks something like
         ;;
         ;; ** TODO [[message:<encoded messageID>][subject]]
         ;;
         ;;    [2021-05-02 Sun 16:22]
         ;;
         ;;    #+begin_quote
         ;;    Unwrapped
         ;;
         ;;    Body
         ;;
         ;;    Text
         ;;    #+end_quote
         (file+headline "~/your-org-todo.org" "Inbox")
         "* TODO %(org-mac-mail-link-get-link-string)

  %U%(org-mac-mail-link-get-body-quote-template-element)" :prepend t :immediate-finish t)))

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;;; Use =C-c C= as your org-capture keybinding
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(eval-after-load 'org
  '(org-defkey org-mode-map (kbd "C-c C") #'org-capture))

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;;; Teach org about opening message links
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(defun org-mac-mail-link-open-link
    (mid _)
  (start-process "open-link" nil "open" (format "message://%%3C%s%%3E"
                                                mid)))

(defun org-mac-mail-link-add-message-links
    ()
  (org-link-set-parameters
   "message" :follow #'org-mac-mail-link-open-link))

(eval-after-load 'org
  '(org-mac-mail-link-add-message-links))